Respirators are used in any environment in which inhalation of the ambient atmosphere is likely to cause harm, for example an atmosphere comprising dangerous chemicals. At their most basic, respirators comprise a mask which is sealed against the face and an air inlet to permit air into the mask so that the user may breathe, with a filter often being used to remove unwanted materials from the inhaled air. There are primarily two types of respirator in use, these being positive pressure respirators and negative pressure respirators. In all respirators it is desirable to prevent the potentially contaminated ambient atmosphere from entering the cavity between the face and the mask. Such ingress may occur if the cavity is at a negative pressure relative to the ambient atmosphere and the mask face-seal fails. Positive pressure respirators try to overcome this problem by providing a steady flow of pressurised air to the user. Battery operated pumps are conventionally used to draw air through the filter to the user. Such respirators are expensive, power-hungry, bulky and susceptible to failure; even though the air is supplied under pressure, the mask cavity may experience a negative pressure relative to the ambient atmosphere under certain circumstances, for instance, when the user speaks or undertakes exercise. Negative pressure regulators such as the respirator worn by British Army personnel, are, in comparison to positive pressure respirators, cheap, lightweight and surprisingly effective. Negative pressure respirators work by the breathing of the user creating a negative pressure in the mask which causes ingress of air from the ambient atmosphere through the filter and into the mask cavity. There is thus no need for bulky air pumps. However, the mask is at a negative pressure relative to the ambient atmosphere for a substantial part of the breathing cycle. Furthermore, misting of the eyepieces that are common in such a mask is a significant problem; such misting arises from exhaled air and from the user sweating into the mask cavity. Misting of the eyepieces may severely compromise the performance of the wearer. Inhaled air may be diverted over the eye pieces into the mouth to reduce the misting problem. However, such an approach has not been entirely successful. The present invention tries to alleviate some of the problems of the prior art.